Multi-Input Multi-Output (MIMO) technology improves efficiency of data transmission and receipt using a multiple transmission antenna and a multiple receipt antenna.
While a receiver receives data through a single path if a single antenna is used, a receiver receives data through multiple paths if a multiple antenna is used. Thus, speed and capacity of data transmission is improved and coverage is enlarged.
Generally, there are an open-loop MINO system in which a transmitter doesn't use feedback information from a receiver and a closed-loop MINO system in which a transmitter uses feedback information from a receiver in multiple antenna systems. In a closed-loop multiple antenna system, a receiver transmits feedback information about channel status to a transmitter and the transmitter detects channel status through it so that performance of a communication system is improved. It is well known that a closed-loop MIMO system may enhance the average user-throughput and cell-edge throughput in the multi-cell environments.
A transmitter multiplies data by precoding matrix and transmits the data. Thus, transmission signal (y) which is transmitted through a multiple antenna by a transmitter is represented as Equation 1.y=Wz  [Equation 1]
Herein, W is an element of a predetermined codebook, and z is a signal outputted from a MIMO encoder.
In a closed-loop multiple antenna system, a receiver selects a precoding matrix from a predetermined codebook and transmits an index of the selected precoding matrix to a transmitter. Then, the transmitter multiplies data by the selected precoding matrix and transmits the data. In a open-loop multiple antenna system, a receiver does not transmit a feedback for a precoding matrix, and a transmitter precodes data using precoding matrixes of a predetermined codebook rotationally.
However, cell-edge area UEs are vulnerable to the inter-cell interference from adjacent cells. In particular, the usage of certain subset of codebook can give a bad influence on the neighboring cell-edge users, which results in the basic PMI (precoding matrix index) restriction/recommendation scheme.
According to the basic PMI restriction/recommendation scheme, some PMIs are restricted or recommended in the whole frames or frequency band. Thus, the throughput of inner-cell UEs is reduced a lot.